BEIJING, March 23 -- At least 34 people have been killed on Tuesday morning in a series of explosions at Brussels airport and a metro station close to the European Union(EU) institutions.

The extremist group Islamic State (IS) has claimed the terror attacks, which put Belgium and other European countries on high alert.

Here is what we know so far:

-- How did the attacks happen?

Two explosions hit the Brussels airport's departure hall approximately at 8 a.m. local time (0700 GMT), killing at least 14 people on the spot. A third bomb was found during a search of the terminal building later in the day and and deactivated.

"There was a small explosion, then a big one," Zach Muzun, a Belgian citizen, told Xinhua.

It took the police and airport staffs around 30 minutes to make preliminary clearance, he said. "When we went out...there was blood everywhere."

Pictures posted on social media showed passengers fleeing the terminal building while smoke came out of smashed windows.

Around one hour after the airport explosions, another blast was reported at the Maelbeek metro station, which lies just several hundred meters from the EU complex.

At least 20 people died in the metro attack and another 106 people were injured, of whom 17 were reported to be in critical condition.

Belgian Prime Minister Charles Michel called it a "black day" for his country.

-- Who carried out the attacks and why?

The IS has claimed responsibility for the attacks in a statement released by the A'maq news agency, which is said to be linked to the extremist group.

Belgian police issued a photograph on Tuesday evening of a man wanted for suspected involvement in the Brussels airport attacks.

The screengrab, taken from video surveillance at the airport's departure hall, shows a man in a white jacket, a white shirt and a dark hat pushing a luggage trolley.

He was filmed alongside two other men wearing black, who are suspected of setting off the explosions in the terminal building.

La Libre Belgique newspaper said the men were wearing a glove on just one hand, which could have been used to hide a detonator.

Belgian federal prosecutor Frederick Van Leeuw confirmed on Tuesday evening that it was "likely that two of the three suspects at the airport killed themselves and the third is on the run."

He said it was too early to confirm a link with the Paris attacks, and the IS claim for responsibility could not yet be verified.

Also on Tuesday evening, the prosecutor said a building in the Brussels district of Schaerbeek was raided by police earlier on the day. Police searching the property found a nail bomb and an IS flag.

In the statement, the IS blamed Belgium for participating in the fight against the IS and said "several fighters" detonated their explosive belts in the airport and in the metro station.

Milan Zitny, a Slovak security analyst, said the Brussels strikes were "probably a reaction" to the capture of Salah Abdeslam, the suspected main mastermind behind the November terror attacks in Paris, which left 130 people dead.

-- How is the security situation in Europe now?

After the Paris terror attacks, Brussels was found to be highly linked to the terror attacks as it is used by terrorists as a hiding place. Tuesday's bombings again showed the security situation in Europe is at high risk.

According to security analyst Jaroslav Nad, the explosions in Brussels earlier in the day were not a surprise, as the Belgian capital is among the cities under an enormous threat of terrorist attacks.

"It's because it's at the heart of Europe, and the community of the powerful converges there," said Nad. He added that such attacks can not be completely prevented.

"It's as if they've attacked the USA and the EU," said Nad.

Following the attacks, Brussels on Tuesday morning raised the threat alert to the highest level 4 from previous level 3. The EU institutions have canceled most meetings and visits scheduled for Wednesday. A nuclear power station in Tihange, southeast Belgium, has been evacuated as a precaution.

The Brussels attacks are "horrific and cowardly... It has struck at the very heart of Brussels. The very heart of Belgium. The very heart of Europe," Dutch Prime Minister Mark Rutte said.

In Paris, security patrols were reinforced in the French capital's two main airports. Additional police forces have been deployed to control trains coming from Belgium.

In the Netherlands, Belgium's northern neighbor, police increased monitoring and surveillance over suspicious situations across the country. There was increased police deployment at train stations in its four major cities and at other train stations with international traffic.

Tightened border controls were introduced at the southern border of the Netherlands, and trains to and from Brussels no longer ride. People travelling to Belgium were stranded at train stations in cities near the Belgian border.

German police have increased security on the borders with Belgium, France, the Netherlands and Luxembourg, as well as German airports and railway stations.

Security measures have also been beefed up in Britain, Denmark, Norway, Finland, Italy, Greece, Poland, Slovakia, Croatia, Romania, Bulgaria, Czech Republic and other countries.

-- International reaction to the attacks:

Leaders from the EU institutions and the EU's 28 member states strongly condemned the deadly attacks in Brussels on Tuesday and expressed their solidarity with Belgian authorities.

The EU heads of state or government and the leaders of the EU institutions published a joint statement hours after explosions killed at least 34 people in Brussels.

"The European Union mourns the victims of today's terrorist attacks in Brussels.," said the leaders in a joint statement.

President of the European Commission Jean-Claude Juncker said the EU institutions will stand united "in the face of terrorism."

"These attacks have hit Brussels today, Paris yesterday - but it is Europe as a whole that has been targeted. The European Union and its institutions stand united in the face of terrorism," Juncker said.

British Prime Minister David Cameronslammed the attacks in Brussels as "appalling and savage," warning that European countries face "a very real terrorist threat."

"These are difficult times, these are appalling terrorists, but we must stand together to do everything we can to stop them and to make sure that although they attack our way of life and attack us because of who we are, we will never let them win," he said.

French President Francois Hollandecalled for "a global response" to terrorism against Europe after the "cowardly and heinous attacks" in Brussels.

China strongly condemned the attacks on Tuesday.

China is willing to enhance cooperation with Belgium and the international community to jointly face the threats and challenges posed by terrorism to safeguard world peace and stability, said Chinese Foreign Ministry spokeswoman Hua Chunying.

U.S. President Barack Obama, while on a visit to Cuba, also expressed his support for the EU, saying "this is yet another reminder that the world must unite."

"We will do whatever is necessary to support our friends and ally Belgium in bringing to justice those who are responsible," he said.